Spectrum of renal pathological findings in patients with chronic myelomonocytic leukemia and kidney injury
Authors: Nate Gipe, Nelson Leung, Terra Lasho, Abhishek Mangaonkar, Hassan Alkhateeb, Aref Al‐Kali, Naseema Gangat, William Hogan, Kristen McCullough, Ayalew Tefferi, Mariam Priya Alexander, Mrinal M. Patnaik
Journal: American Journal of Hematology (2023)

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic/myeloproliferative (MP) overlap neoplasm (MDS/MPN), characterized by sustained peripheral blood monocytosis (absolute monocyte count≥0.5 × 109/L, with monocytes comprising ≥10% of the white blood cell [WBC] differential) and an inherent risk for acute myeloid leukemia (AML) transformation.1, 2 CMML occurs in aging individuals (median age: 73–75 years), arising on a background of clonal hematopoiesis, with TET2 and SRSF2 mutations being early driver events.3, 4 Based on presenting WBC counts CMML can be further classified into MP and myelodysplastic subtypes (MD), with the former defined by a WBC ≥13 × 109/L4. Unlike other MPN and MDS/MPN overlap neoplasms, CMML is associated with significant monocytosis which results in a unique spectrum of end-organ damage.4, 5 Prominent amongst this is a higher incidence of acute kidney injury (AKI-34.9%) and chronic kidney disease (CKD-7.6%), with this impact being statistically higher in patients with MP-CMML versus MD-CMML [AKI 40% vs. 27.8% and CKD 10% vs. 4.2%].5 Further, the prevalence of AKI and CKD has been shown to correlate with peak WBC counts and absolute monocyte counts.5 While the mechanisms of AKI and CKD remain multifactorial, monocytic neoplasms are associated with the overproduction of lysozyme (muramidase), a low-molecular weight (15 000 Da), freely filtered protein that can accumulate in proximal tubular cells resulting in a proximal tubulopathy and subsequent kidney injury (lysozyme-induced nephropathy-LIN).6 Additional mechanisms of kidney injury include direct infiltration of myelomonocytic cells into tubules, presence of extramedullary hematopoiesis (EMH), hyperuricemia-associated nephropathy, autoimmune and paraneoplastic manifestations, and rarely obstructive nephropathy secondary to enlarged lymph nodes and severe hepatosplenomegaly.5, 7 We carried out this study to assess the spectrum of renal pathological findings in CMML patients that underwent kidney biopsies at our institution. The study was carried out after Mayo Clinic IRB approval. The CMML institutional database was queried for patients with an established diagnosis of CMML (World Health Organization and International Consensus Classification 2022) that underwent kidney biopsies at Mayo Clinic.1, 2 The pathology slides were re-reviewed by an expert nephropathologist (MPA). The needle biopsy samples were evaluated by light microscopy, immunofluorescence, and electron microscopy, as previously described.8 Biopsy specimens that only included the renal medulla were excluded for review. LIN was defined by the following pathological criteria: (a) proximal tubular cells having hypereosinophilic granular cytoplasm owing to the presence of abundant intracytoplasmic, PAS (Periodic Acid Schiff)-positive granules, with the absence of atypical crystalline inclusions, (b) ultrastructural evidence of increased number and size of lysosomes, many of which may contain clumped, electron-dense, degenerating cellular organellar debris, in the distribution of the proximal tubular cell cytoplasm and (c) strong diffuse cytoplasmic positivity for lysozyme in abnormal proximal tubules.9, 10 Clinical, molecular and survival data points were retrospectively abstracted. Correlation with serum lysozyme levels was carried out where possible. Descriptive statistical analysis was carried out. Fourteen patients met the criteria for inclusion in the study: median age 67 years (range 47–85), with a male preponderance (70%). The time range for kidney biopsies for these 14 patients was 1999–2022. Twelve (85%) patients met the criteria for MP-CMML, while 12 (85%) patients were classified as having CMML-1 by the 5th edition of the WHO and the 2022 ICC criteria for CMML (Table 1).1, 2 Somatic mutations were assessed in 11 patients, with all 11 (100%) having ASXL1 mutations and with 8 (71%) having oncogenic RAS pathway mutations (CBL-4, NRAS-3, KRAS-1). Indications for kidney biopsies included AKI, CKD, and proteinuria: AKI (n = 4: median serum creatinine 3.4 mg/dl; range 2.12–5.13 mg/dl) and CKD (n = 8: median serum creatinine 1.98 mg/dl; range 1.4–4.01 mg/dl) were common indications and while proteinuria (>150 mg/24-h urine collection) was noted in 6 (46%) assessable patients, 4 (30%) met criteria for nephrotic range proteinuria (>3.5 g/24-h urine collection). The urine sediment analysis was notable for granular and/or hyaline casts in 8 (61.5%) analyzable patients, while red cell casts were not detected. Cumulatively, glomerular sclerosis was seen in a median of 18.5% (range; 0–75%) of analyzed glomeruli, while tubular atrophy was seen in a median of 20% (range; 0–60%) of analyzed tubules. Lysozyme-induced nephropathy was diagnosed in 6 (42.8%) patients, 4 (66%) males. All 6 (100%) patients had MP-CMML and the median WBC at diagnosis of LIN was 21.7 × 109/L (range; 13.2–60.1), while the median serum creatinine at the time of kidney biopsy was 2.12 mg/dl (range; 1.77–5.13 mg/dl). All 5 (100%) assessable (#6 did not have somatic mutational profiling) patients had ASXL1 mutations, while 4 (80%) had oncogenic RAS pathway mutations. Two (33%) patients presented with AKI, while 4 (66%) patients presented with CKD, and only 2 (33%) patients had subnephrotic proteinuria (urine protein levels were not increased in the remaining 4 patients with LIN). Serum lysozyme levels were markedly elevated in 4 (100%) of 4 assessable patients. The findings on light microscopy were characterized by tubular epithelial cell injury. In two cases, many proximal tubular cells had hypereosinophilic granular cytoplasm (Figure 1A) with abundant intracytoplasmic, PAS-positive, silver-negative (Figure 1B), and fuchsinophilic granules (Figure 1C). In three of the six cases (50%) of LIN, there was striking tubular epithelial cell injury and the proximal tubular cells having hypereosinophilic granular cytoplasms and intracytoplasmic PAS-positive granules were less apparent, and consequently not recognized on initial review. Immunohistochemical (IHC) staining for lysozyme revealed strong positivity in the distribution of the proximal tubular cell cytoplasm (Figure 1D). In some cases, there was a uniform positivity of staining, while others, particularly with severe tubular epithelial cell injury, showed patchy staining. Routine immunofluorescence was negative for immunoglobulins and light chains. Ultrastructural studies demonstrated diffuse degenerative changes of the proximal tubular epithelium. The most distinctive finding was abundant membrane-bound cytoplasmic vacuoles within the proximal tubules (Figure 1E) containing electron-dense, degenerating cellular organellar debris, consistent with autophagolysosomes (Figure 1F). The autophagolysosomes were diffusely distributed in proximal tubular epithelia. Patient #6 was identified to have LIN when we retrospectively carried out IHC for lysozyme in 2 additional patients (#3 and #6) with available renal biopsy specimens. Patient #6 had presented with stage IIIa CKD, with biopsy findings of a severe interstitial mixed inflammatory infiltrate suspicious for involvement by CMML. Lysozyme IHC demonstrated lysozyme positivity in the distribution of the proximal tubular cytoplasm. Patient #3 did not have features of LIN and the lysozyme IHC was negative. Renal parenchymal infiltrates by myelomonocytic cells were noted in 3 (21%) patients (Figure 1G). In patient #4, who had concomitant CMML and Erdheim Chester disease (ECD), CD163+ histiocytes were seen on IHC, however, there were also several monocytes that stained positive for lysozyme and OCT2 (Octamer Binding Transcription factor 2; Figure 1H). Negative expression for cyclin D1 in this patient made ECD-related renal infiltration less likely. It was technically challenging to accurately estimate the percentage of renal parenchymal involvement by myelomonocytic cells, given the patchy nature of their infiltration. The glomerular pathology seen in this cohort was variable, however, no salient glomerular pathology was identified in patients with LIN. Patient #14 demonstrated an MPN-related glomerulopathy that was characterized by mesangial matrix expansion, focal mesangiolysis, and ultrastructural evidence of subendothelial lucency and capillary wall remodeling (Figure 1I). Two other cases, both of whom presented with nephrotic syndrome, were diagnosed with minimal change disease, with typical diffuse and extensive foot process effacement in otherwise unremarkable appearing glomeruli. One patient had incidental IgA nephropathy, and another showed a resolving post-infectious glomerulonephritis. At the last follow-up (median 36 months), 4 (28.5%) deaths were documented and there were no patients who needed long-term renal replacement therapy. All 6 patients with LIN, were managed with cytoreduction, with 4 (66%) receiving hydroxyurea (one each with concomitant ruxolitinib and prednisone, respectively) and 2 (33%) receiving decitabine (DNA methyltransferase inhibitor). Cytoreduction was effective in all patients with LIN, reducing the median WBC by 5 × 109/L (range; 3.2–13.1 × 109/L) and the serum creatinine by a median of 0.4 mg/dl (range; 0.3–4.0 mg/dl). Patient #4 with multifactorial acute on CKD with LIN and myelomonocytic infiltrates, did receive hemodialysis for 6 months with ongoing hydroxyurea use and was able to discontinue renal replacement therapy, with a marked improvement in her serum creatinine. There was no evidence for renal tubular acidosis in all six patients with LIN, as in LIN, tubular damage mainly occurs in the proximal tubules. We describe the spectrum of renal pathology in patients with CMML, with LIN, a unique complication seen in patients with “sustained monocytosis” (e.g., sarcoidosis) and “monocytic neoplasms”, being documented in 42.8% of cases.5, 8 As discussed above, progressive leukocytosis and monocytosis are important risk factors for LIN, given that lysozyme is produced by monocytic cells, with all six cases in our series having MP-CMML, with 100% of assessable patients having somatic ASXL1 mutations and 80% oncogenic RAS pathway mutations. We have previously shown the unique biological characteristics of MP-CMML, largely driven by an oncogenic RAS-KMT2A-PLK1 pathway (oncogenic RAS mutations-NRAS, KRAS, CBL, PTPN11, and NF1), with concurrent ASXL1 mutations conferring a very high-risk proliferative phenotype.4, 11 Importantly, while supportive care and cytoreductive therapy did help improve renal function in all patients with LIN, they continued to have CKD, potentially arguing for earlier cytoreduction and renal risk factor mitigation. The high prevalence of CKD in CMML patients (7–10%) often leads to ineligibility for allogeneic hematopoietic stem cell transplantation and clinical trial access, critical treatment modalities for affected patients.5, 12 Other etiologies of kidney disease seen included direct infiltration by myelomonocytic cells in the renal parenchyma, autoimmune disease (minimal change disease) and MPN-associated glomerulopathy. We did not encounter any patients with EMH, urate nephropathy, or obstructive nephropathy. In summary, early workup and referral for evolving renal dysfunction in CMML patients is strongly encouraged with assessment for LIN being high on the differential. We demonstrate that LIN can respond to renal supportive care measures and cytoreduction, arguing for prospective assessments for WBC thresholds at which cytoreductive therapies may be renoprotective. Further work assessing the role of serum lysozyme levels and their correlation with the degree and extent of renal tubular injury are also needed. Current publication is supported in part by grants from “The Henry J. Predolin Foundation for Research in Leukemia, Mayo Clinic, Rochester, MN, USA.” Ayalew Tefferi has served on the ICC panel for the classification of Myeloid Neoplasms. Mrinal Patnaik has served on the 2022 WHO panel for the classification of MDS/MPN Overlap Neoplasms. Mrinal Patnaik has also received research funding from Stem Line Pharmaceuticals and Kura Oncology. Available on contacting [email protected]